Available transmission capacity is an important parameter for controlling use of a communication network. For example, when a device has received a request to transmit a stream of video data, information about available transmission capacity can be used to control the quality level and resulting bandwidth of the stream or to generate an error message if the available transmission capacity is insufficient.
Information about available transmission capacity is not always directly available in every device that uses the network. The network may comprise many layers of software and hardware that make the underlying transmission capacity invisible to devices that operate in higher layers.
US 2007/0115814 describes various techniques for measuring network capacity. One technique involves transmission at increasing data rate until it is detected that the limits of available capacity have been reached, e.g. when packet losses increase. Another technique involves measurement of round trip delay time (RTT) as a function of data rate. In this technique packets are transmitted from a sender to a receiver and the receiver transmits back acknowledgements. The ratio between the amount of data sent and time duration between acknowledgements is used as a measure of capacity.
A further technique involves measurement of “packet pair dispersion”. In this technique a first and second packet are transmitted at time points separated by a first (minimal) time interval and the duration of a second time interval between the time points of arrival of the packets is measured. The packet pair dispersion, i.e. the difference between the durations of the first and second time intervals can be used as a measure of capacity. US 2007/0115814 describes a technique, called “Capprobe” wherein a plurality of such pairs are transmitted and the dispersion is used to select pairs that have been transmitted unhindered by other traffic, using the transmission delays of the packets in the selected pairs as a measure of capacity.
EP 522211 describes a method of determining network bandwidth from a slope of a graph of mean transmission delay versus packet size, which is a straight line. The document notes that the line mage be fragmented when the length of the packet exceeds an internal size of of one or more network links. The document notes that the location of the discontinuity can be used to determine the internal packet size. The document shows the slope disregarding the discontinuity, using successive line fragments with the same slope. The bandwidth is determined from the slope. EP 522211 contains no suggestion that the ratio of mean transmission delay difference and packet size difference between points on mutually opposite sides of the discontinuity has any significance.
Many of the known techniques have the disadvantage that they involve software or hardware that is specifically adapted to implement the technique on both the sender and receiver side. This may not be feasible when the sender and the receiver are controlled by different parties. In that case a technique must be used that involves specific adaptations on one side only, typically on the sender side, using round trip. However, in that case it may be become uncertain how much of the delay must be attributed to transmission